Ethylene elastomer compositions typically contain ethylene, an alpha-olefin such as propylene, and a third component, such as a diene, to facilitate vulcanization. The choice of the third component can greatly influence the nature of branching reactions and the resultant molecular architecture of the polymer. For example, when ethylidene norbornene (ENB) is used as the diene, as is frequently the case, long chain branching through the diene is facilitated through cationic coupling reactions in Ziegler-Natta (Z-N) polymerization, whereas this branching mechanism is not present with metallocene catalysts. The lack of branching found with metallocene-catalyzed polymerization of ENB is due to the absence of Lewis acidity in Z-N catalyst systems that leads to cationic coupling of the pendent double bonds on the ENB molecule.
When other dienes, such as divinyl benzene (DVB) and vinyl norbornene (VNB) are used in place of ENB in the production of ethylene elastomers, the resultant polymers frequently exhibit improved properties, such as enhanced vulcanization characteristics and the ability to undergo cross-linking with peroxides. However, when using metallocene-catalyzed polymerization with dienes, such as DVB and VNB, incorporation of the pendent double bonds into the polymer backbone is facile and leads to excessive amounts of branching and gel formation. Thus, ethylene elastomers containing VNB are generally synthesized using Ziegler-Natta catalysts. However, the resultant polymers still typically retain a high level of long chain branching which, while helpful for processability, may lower the cure rate and degree of curing during subsequent processing of the polymer. There is therefore significant interest in developing a metallocene-catalyzed polymerization process that will allow the production of ethylene elastomers using dienes, such as DVB and VNB, without, or with reduced, gel formation.
Recently, a class of propylene/ethylene copolymers have been commercialized that comprise 5 to 25% by weight of ethylene-derived units and 95 to 75% by weight of propylene-derived units and that contain isotactic propylene sequences through the use of a particular type of metallocene catalyst. These copolymers have shown an attractive balance of flexural modulus, tensile strength and elasticity and should produce useful elastomer compositions when combined with a diene such as VNB. However, the metallocene catalysts used in the synthesis of these copolymers are known to be potent gel producers in the presence of dienes such as VNB, and so it would be desirable to develop a process for producing elastomer compositions based on these propylene/ethylene copolymers in which the problem of gel formation is obviated or minimized.
U.S. Pat. No. 6,096,849 discloses a process for preparing a linear, homogeneous ethylene/alpha-olefin/VNB copolymer in the presence of a single site metallocene catalyst having the structural formula:
wherein M is a transition metal selected from the group consisting of Group 3 and Group 4 metals of the Periodic Table of the Elements; wherein L and L′, independently, are selected from —NR′—, —PR′—, cyclopentadienyl and substituted cyclopentadienyl groups bound in an η5 bonding mode to said metal M; wherein at least one of L and L′ is a cyclopentadienyl or substituted cyclopentadienyl group; wherein Y is a moiety selected from —SiR2′—, —CR2′—, and —CR2′—CR2′—; wherein R′, independently, is selected from hydrogen, alkyl, aryl, silyl, halogeneated alkyl, halogenated aryl, and mixtures thereof, wherein X is selected from hydride, halo, alkyl, aryl, aryloxy, and alkoxy; wherein n is 0, 1 or 2; and wherein the angle formed at the metal center between two L and L′ ligands is between 135 and 105°. Exemplified metallocene catalysts include ethyldiindenylzirconium dichloride, dicyclopentadienylzirconium dichloride, and dicyclopentadienyldimethylsilyl-t-butyl aminotitanium dichloride.
U.S. Pat. No. 6,207,756 discloses a method of making an intimate dispersion having a semicrystalline plastic (SP) component and an amorphous elastomer (AE) component, wherein the method comprises: a) feeding solvent and a first set of monomers in predetermined proportions to a first reactor, b) adding a soluble metallocene catalyst to said first reactor, c) polymerizing the first set of monomers in solution to produce an effluent containing a first polymer, d) feeding the effluent to a second reactor, e) feeding a second set of monomers in predetermined proportions to a second reactor with optionally additional solvent and catalyst, and f) polymerizing the second set of monomers in solution in the presence of the first polymer to produce a second polymer wherein: 1) the first and second set of monomers are chosen from the group ethylene, alpha-olefin, non-conjugated diene, 2) one of the two polymers is an SP having a melting point greater than 60° C., 3) the other polymer is an AE copolymer with 20-70 wt. % ethylene and having a melting point less than 60° C., 4) the first polymer contains less than 0.2 vinyl groups per chain, and 5) the first and second polymer are incompatible and form a two phase mixture. Suitable metallocene catalysts include bridged bisindenyl compounds selected from μ-(CH3)2Si(indenyl)2M(Cl)2, μ-(CH3)2Si(indenyl)2M(CH3)2, μ-(CH3)2Si(tetrahydroindenyl)2M(Cl)2, μ-(CH3)2Si(tetrahydroindenyl)2M(CH3)2, μ-(CH3)2Si(tetrahydroindenyl)2M(CH2CH3)2, and μ-(C6H5)2Si(indenyl)2M(CH3)2; wherein M is chosen from a group consisting of Zr, Hf, and Ti.
U.S. Pat. No. 6,319,998 discloses a method of making a copolymer blend by solution polymerization comprising: (a) feeding a first set of at least two different monomers and a solvent in controlled proportions to a first continuous flow stirred tank reactor; (b) adding a metallocene catalyst to the first reactor; (c) operating the first reactor to polymerize the first set of monomers to produce an effluent containing a first copolymer; (d) feeding the effluent of (c) to a second continuous flow stirred tank reactor; (e) feeding a second set of at least two different monomers in controlled proportions to the second reactor and optionally additional solvent; and (f) operating the second reactor to polymerize the second set of monomers to produce a second copolymer, wherein the monomers of the first and second set of monomers are selected from the group consisting of ethylene, higher alpha-olefins and non-conjugated dienes and wherein the monomer proportions in the first reactor and the second reactor are controlled so that the first copolymer has 0 to 85 wt % ethylene, the second copolymer has 0 to 85 wt % ethylene, and the copolymer blend has 6 to 85 wt % ethylene; and wherein 50-100 wt % of the total amount of catalyst added to all reactors is added to the first reactor. Suitable metallocene catalysts include μ-(CH3)2Si(indenyl)2 Hf(CH3)2, μ-(CH3)2Si[tetramethylcyclopentadienyl][adamantylamido]Ti(CH3)2, and μ-(C6H5)2Si[cyclopentadienyl][flourenyl]Hf(CH3)2.
U.S. Pat. No. 6,806,336 describes a process for solution polymerizing ethylene, propylene and diene having two polymerizable double bonds which comprises: A) reacting in a first step ethylene, propylene and optionally one or more dienes to produce a polymer composition comprising from 0 to less than 1 mol % of diene having two polymerizable double bonds, in the presence of a vanadium based catalyst system; B) reacting in a second step ethylene, higher alpha-olefin comonomer and diene comprising vinyl norbornene in the presence of the same catalyst system, the amount of vinyl norbornene added in the second step being more than 50% a of the total diene added in the first and second step combined; and C) recovering a polymer product having from 0.1 to 0.5 mol % of units derived from vinyl norbornene and a total of no more than 5 mol % diene derived units, from 50 to 90 mol % ethylene derived units, and a balance of propylene derived units.
U.S. Pat. No. 7,135,533 discloses a polymer product which comprises in combination: a) from 50 to 90 mol % of ethylene derived units; b) from 0.1 to 2 mol % of vinyl norbornene (VNB) derived units; c) a balance of higher alpha olefin derived units; d) a branching index of greater than 0.7; and e) an optional amount of ethylidene norbornene (ENB) derived units but less than the amount of VNB.
International Patent Publication No. WO2005/049670 discloses an elastomer comprising: (a) propylene-derived units in an amount of at least 60 wt %, based on the combined weight of components (a), (b), and (c); (b) diene-derived units in an amount within the range of from 0.3 to 10 wt %, based on the combined weight of components (a), (b), and (c); and (c) ethylene-derived units in an amount of at least 6 wt %, based on the combined weight of components (a), (b), and (c); wherein the elastomer has isotactic polypropylene crystallinity, a melting point by DSC equal to or less than 110° C., and a heat of fusion of from 5 J/g to 50 J/g.
In our co-pending, now issued U.S. Pat. No. 7,511,106, we have described a process for producing an ethylene elastomer without the production of gel using VNB as the diene and a specific metallocene catalyst system comprising a bridged bis-indenyl transitional metal compound and a non-coordinating anion (NCA) activator. The bridged bis-indenyl transitional metal compound comprises a cyclopentadienyl (Cp) complex having two ring systems for ligands; wherein these Cp complexes have the general formula: (Cp1R1m)R3(Cp2R2p)MXq; wherein Cp1 of ligand (Cp1R1m) and Cp1 of ligand (Cp2R2p) are the same, R1 and R2 each is, independently, a halogen or a hydrocarbyl, halocarbyl, hydrocarbyl-substituted organometalloid or halocarbyl-substituted organometalloid group containing up to about 20 carbon atoms, m is 1 to 5, p is 1 to 5, and two R1 and/or R2 substituents on adjacent carbon atoms of the cyclopentadienyl ring associated therewith can be joined together to form a ring containing from 4 to about 20 carbon atoms; R3 is a bridging group, such that the number of atoms in the direct chain between the two ligands is 1 to 8, M is a transition metal having a valence of from 3 to 6, from group 4, 5, or 6 of the periodic table of the elements and is in its highest oxidation state, each X is a non-Cp ligand and is, independently, a hydrocarbyl, oxyhydrocarbyl, halocarbyl, hydrocarbyl-substituted organometalloid, oxyhydrocarbyl-substituted organometalloid or halocarbyl-substituted organometalloid group containing up to about 20 carbon atoms, and q is equal to the valence of M minus 2. Examples of suitable transition metal compounds include ethylenebis (indenyl) zirconium dichloride, ethylenebis (tetrahydroindenyl) zirconium dichloride, ethylenebis (indenyl) dimethylzirconium and their hafnium and titanium counterparts.
According to the invention, it has now been found that gel formation in the production of ethylene elastomer compositions using dienes, such as VNB, can be minimized using a metallocene catalyst system comprising a particular bridged, substituted bis-indenyl transitional metal compound and a non-coordinating anion (NCA) activator. The use of this catalyst system allows the production of a wide variety of elastomer compositions, including the semi-crystalline, propylene-rich compositions described above, and also allows higher diene loadings as compared with those normally possible with Ziegler-Natta catalysts. In addition, it is found that the gel suppression characteristics of the catalyst system may be retained even when the catalyst contains substantial amounts of a second transitional metal compound which is known to be a facile and low cost producer of ethylene elastomers because of its high reaction temperature capabilities but which is also known to be potent gel producer in the presence of VNB. Adjusting the ratio of the two catalysts also allows for control of long chain branching over a wide range from linear to gel point.